The present invention relates to an electron gun of a cathode-ray tube used for a television set or a computer display.
It is important to reduce an input capacitance between a cathode and other electrodes, especially a control electrode for adapting the cathode-ray tube to high frequency scanning for a high definition image.
FIGS. 6A and 6B show an example of a structure of the cathode and the control electrode in a conventional electron gun. FIG. 6A is a partial section along a plane perpendicular to the axis of the gun, and FIG. 6B is a partial section along a plane including the axis of the gun. The cathode includes a cylindrical sleeve 1, a cap 2 that closes one end of the sleeve, and an electron emitting substance 3 applied on the outer surface of the cap. The cathode 11 is surrounded by a cylindrical metal shell 5, which is disposed coaxially with the cathode 11. The cylindrical metal shell 5 and the cathode 11 are connected by three metal tabs 4 disposed at an angular spacing of 120 degrees.
The cylindrical metal shell 5 is engaged and fixed in an opening formed in the center portion of an insulator 7. The insulator 7 has an outer metal frame 6, which is attached to the periphery of the insulator 7 and is welded to an inner surface of a cap-shaped control electrode 14. Two brackets 18 are welded to an outer surface of the control electrode 14 at an angular spacing of 180 degrees. Each bracket 18 is embedded in a side of one of the support rods (so-called multiform rods) 13, so that the control electrode 14 is fixed.
The cathode 11 is heated by a heater (not shown in Figures) disposed inside of the cathode 11. The cathode 11 is supported by the insulator 7 indirectly via the cylindrical metal shell 5 and plural tabs 4, as mentioned above, in order to prevent heat of the cathode 11 from escaping via the supporting members for the cathode 11. However, this cylindrical metal shell 5 has an outer surface that faces the inner surface of the control electrode 14 via the insulator 7. Therefore, a substantially large capacitance is formed between the cathode 11 and the control electrode 14.
An improved structure for supporting the cathode is described in Japanese laid-open patent application (Tokkaihei) 3-155026, where the capacitance between the cathode and the control electrode is reduced compared with the above mentioned structure. This supporting structure, which is shown in FIGS. 7A and 7B, uses three metal pins 9 arranged in a circle instead of the cylindrical metal shell 5 used in FIGS. 6A and 6B, and three metal tabs 4. Each tab 4 is welded to the metal pin 9 and the cathode 11 to connect them. Thus, the cathode 11 is supported by the insulator 7' via the tabs 4 and the pins 9. This insulator 7' is different in shape from the insulator 7 in FIGS. 7A and 7B. The insulator 7' has an opening in the center portion and three small holes for fixing the pins 9 around the opening. The diameter of the center opening is a little larger than the outer diameter of the cathode 11. In this supporting structure, the capacitance between the cathode 11 and the control electrode 14 is reduced since the pins 9 have little surface facing the control electrode 14.
However, this supporting structure requires changing the shape of the insulator 7 and more time for manufacturing, so that the cost rises. In addition, the degree of parallelization and the distance between the surface of the electron emitting substance and the control electrode 14 must be controlled precisely. Therefore, the pins 9 have to be fixed to the insulator 7 at the same height and without slant before welding the tabs 4 to the pins 9 precisely. Thus, it is difficult to improve productivity.
The reduction rate of the capacitance by using the plural pins 9 instead of the cylindrical metal shell 5 is about 20-30%, from 5.0 pF to 3.5-4.0 pF, for example. This reduction rate is not enough for high frequency scanning above 100 kHz so as to obtain a high resolution image.
The present invention seeks a large reduction of the capacitance between the cathode and the control electrode without substantial change in the supporting structure for the cathode. It is another purpose of the present invention to reduce capacitance between the cathode and other electrodes such as an accelerating electrode or a focusing electrode, too.